The following relates to the optoelectronic arts. It finds particular application in outdoor illumination using light emitting diodes. However, the following will find more general application in conjunction with illumination generally.
Light emitting diodes are rapidly gaining market share, especially for outdoor lighting applications where their improved robustness against environmental damage and longer operating lifetimes are distinct advantages compared with incandescent and fluorescent light sources. Some rigorous environments in which light emitting diodes have found application include outdoor backlit signage, exterior architecture lighting, illuminated outdoor advertisements, traffic signals, and so forth.
In a common embodiment, the light emitting diode includes a semiconductor light emitting diode chip comprising a layered structure including group III-nitride layers that emits in the blue, violet or ultraviolet, optically coupled with a phosphor or phosphor blend that downconverts some or all of the blue, violet, or ultraviolet emission. In some embodiments, a white emitting phosphor blend is used, and the light emitting diode is designed so that most or all of the chip emission is converted to white by the phosphor blend. In other embodiments, the phosphor or phosphor blend emits yellow, red, or other relatively longer wavelength light that blends with blue or violet chip emission to produce white light. Other materials besides group III-nitrides can be used to fabricate the light emitting chip, and some light emitting diodes output the chip radiation directly without downconversion by a phosphor.
In a usual configuration, the light emitting diode chip is encapsulated by a transparent encapsulant, and the phosphor or phosphor blend is disposed on the chip, dispersed in the encapsulant, disposed on top of the encapsulant, or otherwise arranged to receive and downconvert the chip emission. The encapsulant provides environmental protection, provides a substrate or matrix for supporting the phosphor or phosphor blend, and optionally acts as a refractive or diffractive optical element, for example by being formed into a lens.
Epoxy is a known transparent encapsulant material suitable for use in light emitting diodes. However, typical epoxies have been found to degrade in the presence of the operating light emitting diode chip. The degradation manifests as a “browning” or other discoloration of the epoxy, which acts as a light absorber and substantially reduces light output efficiency. This degradation is particularly problematic when the chip emits in the violet or ultraviolet range, although epoxy degradation due to longer wavelength emission and or heating caused by the operating chip is also of concern.
Another known transparent encapsulant is silicone, which shows greater robustness to exposure to an operating light emitting diode chip, and generally does not “brown” or otherwise discolor over time in such an environment. However, silicone does not provide an effective hermetic sealing of the encapsulated light emitting diode chip. This lack of hermetic sealing can lead to environmental exposure of the chip, lead frame, or other components. As a result, light emitting diodes employing silicone encapsulation have lower yield during manufacturing due to higher impact on the chip of processes such as reflow soldering, and are expected to have higher failure rates when used outdoors or in other rigorous environments. The limited sealing ability of silicone is particularly problematic in surface mount light emitting diodes in which the chip is mounted to a circuit board directly or via the intermediary of a small slug or other submount. In such devices, sealing the gap or interface between the circuit board and the chip is especially problematic.
In another approach, a transparent silicone encapsulant is used, and a conformal spray coating is applied over the silicone. The spray coating can provide additional sealing, but is prone to lower yields and reliability problems due to non-uniformities sometimes observed in spray coatings. Such spray coatings sometimes have small voids, holes, thin regions, or so forth that compromise the intended hermetic sealing. Epoxy spray coatings are expected to be less prone to degradation due to the separation from the chip—however, degradation of the epoxy spray coating is nonetheless sometimes observed. Silicone spray coatings suffer from the same deficiencies as the silicone encapsulant itself, and accordingly typically provide less than ideal hermetic sealing.
In another approach, a separate enclosure or cover is provided. For example, Aanegola et al., U.S. Publ. Appl. 2005/0239227 A1, discloses a surface mount light emitting diode disposed on a circuit board and encapsulated by silicone, over which is placed a dome-shaped cover of glass or another transparent material. In some disclosed embodiments, the dome-shaped cover is hermetically sealed with the circuit board. A phosphor or phosphor blend may be disposed on the dome-shaped cover, in the silicone, or over the chip. This approach has numerous advantages.
However, placement of the dome-shaped cover over the silicone encapsulant and sealing of the cover to the circuit board adds manufacturing complexity. Thermal expansion or contraction due to large outdoor temperature changes can produce overpressure or underpressure in the sealed volume defined by the circuit board and the dome-shaped cover. Other enclosure or covering approaches such as covering by a separate lensing element, enclosure in a sealed traffic light housing, or so forth similarly add manufacturing complexity and raise the possibility of temperature-related overpressures or underpressures. The enclosure or cover also does not provide protection against damage during reflow soldering or other manufacturing processes that may precede addition of the cover or enclosure.